Car heater acting really weird. I already know my heater core is plugged so this is really just more of a curiosity thing but the driver side vents blow Luke warm air on low but the passenger side vents blow cold. What would cause this?

Another possibility is one or more of the vent doors that direct the path of the hvac air flow are not working. On newer cars those are often small electro-mechanical actuators, servos, and who’s failures – not just in minis but many makes of cars – are a frequent source of complaints here. I wouldn’t make sense to presume that’s the problem of course until the heater core is working.

Sometimes a blocked heater core can be reversed flushed and get it working again. A diyer could figure out a way to hook up a garden hose so the flow – accessed at the firewall – ran through the heater core backward. Could be a big, drenching, messy job though, especially if done in the dead of winter…